1. Field of the Invention
The present invention relates to a position detecting arrangement for detecting the position of a movable member relative to a reference position and, more particularly, the present invention relates to an electrically moved zoom lens of a camera having a lens position detecting device for electrically detecting changes in the focal length of the zoom lens associated with a zooming operation.
2. Discussion of Background Information
In a camera equipped with a zoom lens, it is necessary to know the F-number of the zoom lens so that a proper exposure can be calculated. In addition, if the camera has a power zoom lens, the focal length setting must be known so that the camera can control the zooming operation.
A conventional solution to the above problems has been to provide the zoom lens with a code plate located on a cam ring for driving the zoom lens for integral rotation therewith. In such an arrangement, brushes are provided in slidable contact with the code plate for generating a position code that corresponds to the focal length of the lens in dependence upon the zoom code detected as a result of a conductive state between a ground brush that is normally in contact with the conductive pattern of the code plate and code brushes that come into contact with the code plate during switching between conductive areas and non-conductive areas of the code plate as the cam ring is rotated.
Each code brush generates a 1-bit signal that depends on the conductive and non-conductive state of the code plate so that when several such brushes are employed, a multi-bit zoom code signal is produced. However, since the brushes and the code plate form a mechanical configuration, the code brushes can become separated from the code plate and not make proper electrical contact due to external causes, such as vibrations or dust, dirt, or other foreign matter on the contact surfaces. In such a case, the resulting position code that is calculated from the detected zoom code will not correspond to the actual lens position.
In addition, in determining the position code of the lens (i.e., the focal length of the lens), the exposure control is likely to be improperly set if the position code is not accurately stepped. For example, in a camera having a lens with a range of approximately 35 mm to 60 mm, the camera should preferably have about fifteen position codes to ensure that the exposure is properly calculated.
In a conventional camera, one zoom code corresponds to one position code. That is, an absolute code system is used in which each step of the zoom code has a unique value that is specific to that step and, therefore, is specific to a particular position of the zoom lens.
However, a problem associated with such an arrangement is that if the zoom code is set as an absolute code over the entire range of the zoom lens, the number of brushes that are used in association with the code plate must be increased, resulting in a more complicated code plate structure.
For instance, if information in fifteen steps is to be detected, at least four-bits of information are necessary for the generation of the zoom code. Thus, five brushes would be required, which includes one brush for ground, to cooperate with the code plate. While it is desirable to minimize the number of brushes that are required, so as to produce a more compact camera, it is not practical to reduce the number of steps of the position code.
Some electronically controlled cameras include macro and normal photographing functions which are switchable between a macro mode and a normal mode by a manual switch operation. In either mode, photographing is accomplished by autofocus in accordance with distance measuring information. A particular camera of this type is one in which the normal mode is constituted by a zoom mode and the camera is switchable between the zoom mode and the macro mode, whereby the lens moves to and stops at a macro position when the macro mode is selected by means of the manual switch and whereby the zoom lens moves to and stops within the zoom range when the zoom mode is selected by means of the manual switch. When the lens is in the zoom mode, operation of a wide switch or a tele switch moves the zoom lens accordingly toward the wide extremity of the zoom range or toward the tele extremity of the zoom range, respectively.
The distance between the subject to be photographed and the camera, as well as the subject light intensity, are measured when the zoom lens is set to a desired position and the shutter button is operated. Based upon the result of the distance measurement, autofocus is effected in both the macro mode and the zoom mode. Photographing is also done in dependence upon the result of the light measurement.
In conventional electronically controlled cameras, if the subject is found to lie within a range where focusing is impossible while in the macro mode, which is set by the manual switch thereby placing the zoom lens in the macro position, photographing can only be effected by manually switching from the macro mode to the zoom mode. This requirement, of course, creates a considerable inconvenience since a quick response is virtually impossible to achieve which can be necessary to obtain the photograph which had been composed.
Applicants have recognized, therefore, that when the distance measurement is made when the lens is placed in the macro position by operation of the manual switch, it would be desirable to automatically effect switching from the macro mode to the zoom mode so that a subject which cannot be photographed in the macro mode can be automatically photographed in the zoom mode.
A problem encountered with such automatic switching, however, is that if the lens is shifted from a macro position to a normal position (zoom position, e.g.), a desired photograph which had been composed in the macro mode cannot be obtained due to the parallax which exists between the lens and the viewfinder of the camera.
Another problem encountered with automatic cameras which are designed to perform autofocus photographing in either a macro mode or a zoom mode is the possibility of not being able to accomplish photographing in either mode due to subject distance measurement errors.
For example, assume that autofocus is designed to be performed in the zoom mode for a subject distance range of 0.90 meters to infinity (i.e., 0.90 m-.infin.). and in the macro mode for a subject distance range of 0.50 m-0.90 m. Further, assume that the lens is set to the zoom position by means of the manual switch and that the distance measurement made by the camera is 0.89 m for an object positioned at that distance. In such a case, the object distance is outside of the aforementioned focusing range, so that a photograph cannot be obtained. In this situation, conventional cameras provide the photographer with a warning indication that a properly focused photograph cannot be obtained, instructing the photographer to switch to the macro mode, or a release lock is operated to prevent release of the shutter, even if the shutter button is operated.
In response the photographer switches the camera to the macro mode by means of the manual switch. However, for the object positioned at the same distance of 0.89 m, it is conceivable that the distance measurement made by the camera can erroneously result in a value of 0.91 m. In this case, the photographer is warned that a properly focused photograph cannot be made, or is prevented from releasing the shutter due to a release lock operation, even in the macro mode.
This predicament is, of course, not desirable and is particularly serious when the automatic camera is provided with a release lock capability which renders impossible the taking of photographs within a certain subject distance range.
Another problem relates to cameras having a built-in strobe unit. A recent trend is to provide a camera with a so-called auto strobe capability in which the camera judges whether to cause the strobe to flash automatically when an exposure value is below a preset level.
Such auto strobe cameras are typically designed to illuminate a red lamp indicator during the charging operation so that the user is warned to wait until the charging is finished, or merely to warn the user that the charging is not yet done and that the strobe is not ready when the shutter button is depressed.
An ideal arrangement would be to provide an auto strobe camera in which the photographer is allowed to shoot pictures without having to pay attention to the charging of the strobe. In practice, a red lamp indicator is either kept lit during the charging cycle regardless of whether the photographer actually intends to take a picture, or turned on if the strobe is not ready at the time when the shutter button is operated and kept lit regardless of whether the charging of the strobe has been finished. That is, the problem has been such that the red lamp indicator is illuminated to inform the photographer to wait for shooting, even when the photographer has no intention in pressing the shutter button for shooting, or that there is no indication of when shooting is ready, although the photographer is waiting for the strobe to become ready.
Still another problem with prior electronically controlled cameras concerns the liquid crystal display which indicates the different operating states of the camera. Such an indicator provides information on the frame number of the film loaded in the camera, the focal length of the zoom lens, the picture taking mode of the camera, the status of the battery in the camera, and the status of the film loading operation, for example.
However, only a limited amount of space is available on the camera body for such an indicator. If one attempts to provide a display panel that can display all of the desired information at one time, each indicator on the display panel would tend to be so small as to be illegible. Thus, some cameras do not display all of the information, and merely display the information that the camera manufacturer considers to be the most important to the average photographer. However, this practice deprives some photographers who prefer to have all necessary information available to them prior to taking a photograph.
Some electronically controlled cameras are equipped with a back cover closing detector switch which detects the closing of the back cover after the cover is opened and film is loaded in the camera. In response to the detection of the closing of the back cover, the main CPU of the camera drives the film advance motor to effect the "blank-shot" advance of a certain number of film frames.
Problems exist with back cover closing switches, particularly with regard to manufacturing difficulties and operational unreliability. Applicants have determined that it is preferable to use a software substitute for minimizing the number of parts such as switches that are required for film travel for enabling an improved assembling efficiency, while also making the operation of the blank-shot film advance more reliable.